magnetic compass adjustment manual

8/12/2019 Magnetic Compass Adjustment Manual

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HANDBOOK OF MAGNETIC

COMPASS ADJUSTMENT

NATIONAL

GEOSPATIAL-INTELLIGENCE AGENCYBETHESDA, MD

2004

(Formerly Pub.No. 226)

AS ORIGINALLY PUBLISHED BY

DEFENSE MAPPING AGENCYHYDROGRAPHIC/TOPOGRAPHIC CENTER

WASHINGTON, D.C.

1980


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INTRODUCTION

This document has been prepared in order to present all pertinent information regarding the practical procedures of

magnetic compass adjustment in one text. As such, it treats of the basic principles of compass deviations and their correction,

and not of the details of particular compass equipment.

Although this text is presented as a systematic treatise on compass adjustment, ship's personnel who are inexperienced with

compass correction will find sufficient information in Chapters I and XIV to eliminate compass errors satisfactorily without

intensive study of the entire text. Reference should also be made to figure 318 for condensed information regarding the

various compass errors and their correction.

In this handbook, the term compass adjustmentrefers to any changes of permanent magnet of soft iron correctors whereby

normal compass errors are reduced. The term compass compensationrefers to any change in the current supplied to compass

compensating coils whereby the errors due to degaussing are reduced.

The basic text is the outgrowth of lecture notes prepared by Nye S. Spencer and George F. Kucera while presenting courses

of instruction in adjustment and compensation during World War II at the Magnetic Compass Demonstration Station, Naval

Operating Base, Norfolk, Virginia.

CHAPTER I

PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT (CHECK-OFF LIST)

NOTE: If the magnetic adjustment necessitates (a) movement of degaussing compensating coils, or (b) a change of Flinders

bar length, the coil compensation must be checked. Refer to Chapter XIV.

101. Dockside tests and adjustments1. Physical checks on the compass and binnacle.

(a) Remove any bubbles in compass bowl (article 402).(b) Test for moment and sensibility of compass needles (article 403).(c) Remove any slack in gimbal arrangement.(d) Magnetization check of spheres and Flinders bar (article 404).(e) Alignment of compass with fore-and-aft line of ship (article 405).(f) Alignment of magnets in binnacle.(g) Alignment of heeling magnet tube under pivot point of compass.(h) See that corrector magnets are available.

2. Physical checks of gyro, azimuth circle, and peloruses.(a) Alignment of all gyro repeater peloruses or dial peloruses with fore-and-aft line of ship (article 405).(b) Synchronize gyro repeaters with master gyro.(c) Make sure azimuth circle and peloruses are in good operating condition.*

3. Necessary data.(a) Past history or log data which might establish length of Flinders bar (articles 407 and 607)(b) Azimuths for given date and observer's position (Chapter VIII).(c) Ranges or distant objects in vicinity (local charts).*(d) Correct variation (local charts).(e) Degaussing coil current settings for swing for residual deviations after adjustment and compensation (ship's

Degaussing Folder).

4. Precautions.(a) Determine transient deviations of compass from gyro repeaters, doors, guns, etc. (Chapter X).(b) Secure all effective magnetic gear in normal seagoing position before beginning adjustments.(c) Make sure degaussing coils are secured before beginning adjustments. Use reversal sequence, if necessary.(d) Whenever possible, correctors should be placed symmetrically with respect to the compass (articles 318

and 613).

5. Adjustments.(a) Place Flinders bar according to best available information (articles 407, 608 and 609).(b) Set spheres at midposition, or as indicated by last deviation table.(c) Adjust heeling magnet, using balanced dip needle if available (Chapter XI).

*Applies when system other than gyro is used as heading reference


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102. Adjustments at sea.These adjustments are made with the ship on an even keel and after steadying on each heading.

When using the gyro, swing from heading to heading slowly and check gyro error by sun's azimuth or ranges on each heading

if desired to ensure a greater degree of accuracy (article 706). Be sure gyro is set for the mean speed and latitude of the

vessel. Note all precautions in article 101(4) above. "OSCAR QUEBEC" international code signal should be flown to

indicate such work is in progress. Chapter VII discusses methods for placing the ship on desired headings.

1. Adjust the heeling magnet, while the ship is rolling, on north and south magnetic heading until the oscillationsof the compass card have been reduced to an average minimum. (This step is not required if prior adjustment

has been made using a dip needle to indicate proper placement of the heeling magnet.)

2. Come to an east (090) cardinal magnetic heading. Insert fore-and-aft B magnets, or move the existing Bmagnets, in such a manner as to remove all deviation.3. Come to a south (180) magnetic heading. Insert athwartship Cmagnets, or move the existing Cmagnets, in

such a manner as to remove all deviation.

4. Come to a west (270) magnetic heading. Correct half of any observed deviation by moving the Bmagnets.5. Come to a north (000) magnetic heading. Correct half of any observed deviation by moving the Cmagnets.

(The cardinal heading adjustments should now be complete.)

6. Come to any intercardinal magnetic heading, e.g. northeast (045). Correct any observed deviation by movingthe spheres in or out.

7. Come to the next intercardinal magnetic heading, e.g. southeast (135). Correct half of any observed deviationby moving the spheres. (The intercardinal heading adjustments should now be complete, although more

accurate results might be obtained by correcting the D error determined from the deviations on all four

intercardinal heading, as discussed in article 501.)

8.

Secure all correctors before swinging for residual deviations.9. Swing for residual undegaussed deviations on as many headings as desired, although the eight cardinal andintercardinal headings should be sufficient.

10. Should there still be any large deviations, analyze the deviation curve to determine the necessary correctionsand repeat as necessary steps 1 through 9 above (Chapter V).

11. Record deviations and the details of corrector positions on standard Navy Form NAVSEA 3120/4 and in theMagnetic Compass Record NAVSEA 3120/3 (article 901).

12. Swing for residual degaussed deviationswith the degaussing circuits properly energized (Chapter XIV).13. Record deviations for degaussed conditions on standard Navy Form NAVSEA 3120/4.

103.The above check-off list describes a simplified method of adjusting compasses, designed to serve as a simple workable

outline for the novice who chooses to follow a step-by-step procedure. The "Dockside Tests and Adjustments" are essential

as a foundation for the "Adjustments at Sea", and if neglected may lead to spurious results or needless repetition of the

procedure at sea. Hence, it is strongly recommended that careful considerations be given these dockside checks prior to

making the final adjustment so as to allow time to repair or replace faulty compasses, anneal or replace magnetized spheres or

Flinders bar, realign binnacle, move gyro repeater if it is affecting the compass, or to make any other necessary preliminary

repairs. It is further stressed that expeditious compass adjustment is dependent upon the application of the various correctors

in a logical sequence so as to achieve the final adjustment with a minimum number of steps. This sequence is incorporated in

the above check-off list and better results will be obtained if it is adhered to closely. Figure 318 presents the various compass

errors and their correction in condensed form. The table in figure 103 will further clarify the mechanics of placing the

corrector magnets, spheres, and Flinders bar. Chapter IV discusses the more efficient and scientific methods of adjusting

compasses, in addition to a more elaborate treatment of the items mentioned in the check-off list. Frequent, careful

observations should be made to determine the constancy of deviations and results should be systematically recorded.

Significant changes in deviation will indicate the need for readjustment.

To avoid Gaussin error(article 1003) when adjusting and swinging ship for residuals, the ship should be steady on the

desired heading for at least 2 minutes prior to observing the deviation.


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Figure 205 Terrestrial magnetism

206.Inasmuch as the magnetic poles of the earth are not coincidental with the geographic poles, it is evident that a compass

needle in line with the earth's magnetic field will not indicate true north, but magnetic north. The angular difference between

the true meridian (great circle connecting the geographic poles) and the magnetic meridian (direction of the lines of magnetic

flux) is called variation. This variation has different values at different locations on the earth. These values of magnetic

variation may be found on the compass rose of navigational charts. The variation for most given areas undergoes an annual

change, the amount of which is also noted on all charts. See figure 206.

Figure 206 Compass rose showing variation and annual change


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207. Ship's magnetism.A ship, while in the process of being constructed, will acquire magnetism of a permanent nature

under the extensive hammering it receives in the earth's magnetic field. After launching, the ship will lose some of this

original magnetism as a result of vibration, pounding, etc., in varying magnetic fields, and will eventually reach a more or

less stable magnetic condition. This magnetism which remains is the permanent magnetism of the ship.

208.The fact that a ship has permanent magnetism does not mean that it cannot also acquire induced magnetism when placed

in a magnetic field such as the earth's field. The amount of magnetism induced in any given piece of soft iron is dependent

upon the field intensity, the alignment of the soft iron in that field, and the physical properties and dimensions of the iron.This induced magnetism may add to or subtract from the permanent magnetism already present in the ship, depending on

how the ship is aligned in the magnetic field. The softer the iron, the more readily it will be induced by the earth's magnetic

field and the more readily it will give up its magnetism when removed from that field.

209.The magnetism in the various structures of a ship which tends to change as a result of cruising, vibration, or aging, but

does not alter immediately so as to be properly termed induced magnetism, is called subpermanent magnetism. This

magnetism, at any instant, is recognized as part of the ship's permanent magnetism, and consequently must be corrected as

such by means of permanent magnet correctors. This subpermanent magnetism is the principal cause of deviation changes on

a magnetic compass. Subsequent reference to permanent magnetism in this text will refer to the apparent permanent

magnetism that includes the existing permanent and subpermanent magnetism at any given instant.

210.A ship, then, has a combination of permanent, subpermanent, and induced magnetism, since its metal structures are of

varying degrees of hardness. Thus, the apparent permanent magnetic condition of the ship is subject to change fromdeperming, excessive shocks, welding, vibration, etc.; and the induced magnetism of the ship will vary with the strength of

the earth's magnetic field at different magnetic latitudes, and with the alignment of the ship in that field.

211. Resultant induced magnetism from earth's magnetic field. The above discussion of induced magnetism and

terrestrial magnetism leads to the following facts. A long thin rod of soft iron in a plane parallel to the earth's horizontal

magnetic field,H, will have a red (north) pole induced in the end toward the north geographic pole and a blue (south) pole

induced in the end toward the south geographic pole. This same rod in a horizontal plane but at right angles to the horizontal

earth's field would have no magnetism induced in it, because its alignment in the magnetic field is such that there will be no

tendency toward linear magnetization and the rod is of negligible cross section. Should the rod be aligned in some horizontal

direction between those headings that create maximum and zero induction, it would be induced by an amount that is a

function of the angle of alignment. If a similar rod is placed in a vertical position in northern latitudes so as to be aligned with

the vertical earth's fieldZ, it will have a blue (south) pole induced at the upper end and a red (north) pole induced at the lower

end. These polarities of vertical induced magnetization will be reversed in southern latitudes. The amount of horizontal or

vertical induction in such rods, or in ships whose construction is equivalent to combinations of such rods, will vary with the

intensity ofHandZ, heading, and heel of the ship.


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CHAPTER III

THEORY OF MAGNETIC COMPASS ADJUSTMENT

301. Magnetic adjustment.The magnetic compass, when used on a steel ship, must be so corrected for the ship's magnetic

conditions that its operation approximates that of a nonmagnetic ship. Ship's magnetic conditions create deviations of the

magnetic compass as well as sectors of sluggishness and unsteadiness. Deviation is defined as deflection of the card (needles)

to the right or left of the magnetic meridian. Adjustment of the compass is the arranging of magnetic and soft iron correctorsabout the binnacle so that their effects are equal and opposite to the effects of the magnetic material in the ship, thus reducing

the deviations and eliminating the sectors of sluggishness and unsteadiness.

The magnetic conditions in a ship which affect a magnetic compass are permanent magnetism and induced magnetism, as

discussed in Chapter II.

302. Permanent magnetism and its effects on the compass.The total permanent magnetic field effect at the compass may

be broken into three components mutually 90 apart, as shown in figure 302a. The effect of the vertical permanent

component is the tendency to tilt the compass card and, in the event of rolling or pitching of the ship to create oscillating

deflections of the card. Oscillation effects that accompany roll are maximum on north and south compass headings, and those

that accompany pitch are maximum on east and west compass headings. The horizontal Band Ccomponents of permanent

magnetism cause varying deviations of the compass as the ship swings in heading on an even keel. Plotting these deviations

against compass heading will produce sine and cosine curves, as shown in figure 302b. These deviation curves are called

semicircular curves because they reverse direction in 180.

Figure 302a Components of permanent magnetic Figure 302b Permanent magnetic deviation effects

field at the compass

303.The permanent magnetic semicircular deviations can be illustrated by a series of simple sketches, representing a ship on

successive compass headings, as in figures 303a and 303b.

304. The ships illustrated in figures 303a and 303b are pictured on cardinal compass headings rather than on cardinal

magnetic headings, for two reasons:

(1) Deviations on compass headings are essential in order to represent sinusoidal curves that can be analyzedmathematically. This can be visualized by noting that the ship's component magnetic fields are either in line with or

perpendicular to the compass needles only on cardinal compass headings.

(2) Such a presentation illustrates the fact that the compass card tends to float in a fixed position, in line with themagnetic meridian. Deviations of the card to right or left (east or west) of the magnetic meridian result from the

movement of the ship and its magnetic fields about the compass card.


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Figure 305 General force diagram

306. Induced magnetism and its effects on the compass. Induced magnetism varies with the strength of the surrounding

field, the mass of metal, and the alignment of the metal in the field. Since the intensity of the earth's magnetic field varies

over the earth's surface, the induced magnetism in a ship will vary with latitude, heading, and heel of the ship.

307.With the ship on an even keel, the resultant vertical induced magnetism, if not directed through the compass itself, will

create deviations that plot as a semicircular deviation curve. This is true because the vertical induction changes magnitude

and polarity only with magnetic latitude and heel and not with heading of the ship. Therefore, as long as the ship is in the

same magnetic latitude, its vertical induced pole swinging about the compass will produce the same effect on the compass as

a permanent pole swinging about the compass. Figure 307a illustrates the vertical induced poles in the structures of a ship.

Figure 307a Ship's vertical induced magnetism Figure 307b Induced magnetic deviation effects

Generally, this semicircular deviation will be a Bsine curve, as shown in figure 307b, since most ships are symmetrical

about the centerline and have their compasses mounted on the centerline. The magnitude of these deviations will change with

magnetic latitude changes because the directive force and the ship's vertical induction both change with magnetic latitude.

308. The masses of horizontal soft iron that are subject to induced magnetization create characteristic deviations, as indicated

in figure 307b. The Dand Edeviation curves are called quadrantal curves because they reverse polarity in each of the four

quadrants.


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309. Symmetrical arrangements of horizontal soft iron may exist about the compass in any one of the patterns illustrated in

figure 309.

Figure 309 Symmetrical arrangements of horizontal soft iron

310.The deviation resulting from the earth's field induction of these symmetrical arrangements of horizontal soft iron are

illustrated in figure 310, showing the ship on various compass headings. The other heading effects may be similarly studied.

Such a Ddeviation curve is one of the curves in figure 307b. It will be noted that these Ddeviations are maximum on the

intercardinal headings and zero on the cardinal headings.

Figure 310 Effects of symmetrical horizontal Dinduced magnetism


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311. Asymmetrical arrangements of horizontal soft iron may exist about the compass in a pattern similar to one of those in

figure 311.

Figure 311 Asymmetrical arrangements of horizontal soft iron

312.The deviations resulting from the earth's field induction of these asymmetrical arrangements of horizontal soft iron are

illustrated in figure 312, showing the ship on different compass headings. The other heading effects may be similarly studied.

Such an Edeviation curve is one of the curves in figure 307b. It will be observed that these Edeviations are maximum on

cardinal headings and zero on the intercardinal headings.

Figure 312 Effects of asymmetrical horizontal Einduced magnetism

313.The quadrantal deviations will not vary with latitude changes, because the horizontal induction varies proportionally

with the directive force, H.

314. The earth's field induction in certain other asymmetrical arrangements of horizontal soft iron creates a constant A

deviation curve. The magnetic A and E errors are of smaller magnitude than the other errors, but, when encountered, are

generally found together, since they both result from asymmetrical arrangements of horizontal soft iron. In addition to this

magnetic Aerror, there are constant Adeviations resulting from: (1) physical misalignments of the compass, pelorus, or gyro;

(2) errors in calculating the sun's azimuth, observing time, or taking bearings.


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319. Compass operation.Figure 319 illustrates a point about compass operation. Not only is an uncorrected compass subject

to large deviations, but there will be sectors in which the compass may sluggishly turn with the ship and other sectors in

which the compass is too unsteady to use. These performances may be appreciated by visualizing a ship with deviations as

shown in figure 319, as it swings from west through north toward east. Throughout this easterly swing the compass deviation

is growing more easterly; and, whenever steering in this sector, the compass card sluggishly tries to follow the ship.

Similarly, there is an unsteady sector from east through south to west. These sluggish and unsteady conditions are always

characterized by the positive and negative slopes in a deviation curve. These conditions may also be associated with the

maximum and minimum directive force acting on the compass (see article 305). It will be observed that the maximum

deviation occurs at the point of average directive force and that the zero deviations occur at the points of maximum andminimum directive force.

Figure 319 Uncompensated deviation curve

320.Correction of compass errors is generally achieved by applying correctors so as to reduce the deviations of the compass

for all headings of the ship. Correction could be achieved, however, by applying correctors so as to equalize the directive

forces across the compass position for all headings of the ship. The deviation method is more generally used because it

utilizes the compass itself to indicate results, rather than some additional instrument for measuring the intensity of magnetic

fields.

321.Occasionally, the permanent magnetic effects at the location of the compass are so large that they overcome the earth's

directive force, H. This condition will not only create sluggish and unsteady sectors, but may even freeze the compass to one

reading or to one quadrant, regardless of the heading of the ship. Should the compass be so frozen, the polarity of themagnetism which must be attracting the compass needles is indicated; hence, correction may be effected simply by the

application of permanent magnet correctors in suitable quantity to neutralize this magnetism. Whenever such adjustments are

made, it would be well to have the ship placed on a heading such that the unfreezing of the compass needles will be

immediately evident. For example, a ship whose compass is frozen to a north reading would require fore-and-aft Bcorrector

magnets with the red ends forward in order to neutralize the existing blue pole that attracted the compass. If made on an east

heading, such an adjustment would be practically complete when the compass card was freed so as to indicate an east

heading.

322. Listed below are several reasons for correcting the errors of the magnetic compass:

(1) It is easier to use a magnetic compass if the deviations are small.(2) Although a common belief is that it does not matter what the deviations are, as long as they are known, this is in

error inasmuch as conditions of sluggishness and unsteadiness accompany large deviations and consequently make

the compass operationally unsatisfactory. This is the result of unequal directive forces on the compass as the shipswings in heading.

(3) Furthermore, even though the deviations are known, if they are large they will be subject to appreciable change withheel and latitude changes of the ship.

323. Subsequent chapters will deal with the methods of bringing a ship to the desired heading, and the methods of isolating

deviation effects and of minimizing interaction effects between correctors. Once properly adjusted, the magnetic compass

deviations should remain constant until there is some change in the magnetic condition of the vessel resulting from magnetic

treatment, shock from gunfire, vibration, repair, or structural changes. Frequently, the movement of nearby guns, doors, gyro

repeaters, or cargo affects the compass greatly.


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CHAPTER IV

PRACTICAL PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT

NOTE: If the adjuster is not familiar with the theory of magnetic effects, the methods of analyzing deviation curves, and the

methods of placing a ship on any desired heading, it is recommended to review Chapters II, V and VII, respectively, before

beginning adjustment.

401. Dockside tests and adjustments. Chapter I, "Procedures for Magnetic Compass Adjustment" is, in general, self-

explanatory, and brings to the attention of the adjuster many physical checks which are desirable before beginning an

adjustment. The theoretical adjustment is based on the premise that all the physical arrangements are perfect, and much time

and trouble will be saved while at sea if these checks are made before attempting the actual magnet and soft iron corrector

adjustments. A few of these checks are amplified below.

402.Should the compass have a small bubble, compass fluid may be added by means of the filling plug on the side of the

compass bowl. If an appreciable amount of compass liquid has leaked out, a careful check should be made on the condition

of the sealing gasket and filling plug. U.S. Navy compass liquid may be a mixture of 45% grain alcohol and 55% distilled

water, or a kerosene-type fluid. These fluids are NOT interchangeable.

403. The compass should be removed from the ship and taken to some place free from all magnetic influences except the

earth's magnetic field for tests of moment and sensibility. These tests involve measurements of the time of vibration and the

ability of the compass card to return to a consistent reading after deflection. These tests will indicate the condition of the

pivot, jewel, and magnetic strength of the compass needles. (See NAVSEA 3120/3 for such test data on standard Navy

compass equipment.)

404. A careful check should be made on the spheres and Flinders bar for residual magnetism. Move the spheres as close to

the compass as possible and slowly rotate each sphere separately. Any appreciable deflection (2 or more) of the compass

needles resulting from this rotation indicates residual magnetism in the spheres. This test may be made with the ship on any

steady heading. The Flinders bar magnetization check is preferably made with the ship on steady east or west compass

heading. To make this check: (a) note the compass reading with the Flinders bar in the holder; (b) invert the Flinders bar in

the holder and again note the compass reading. Any appreciable difference (2 or more) between these observed readings

indicates residual magnetism in the Flinders bar. Spheres or Flinders bars that show signs of such residual magnetism should

be annealed, i.e. heated to a dull red and allowed to cool slowly.

405. Correct alignment of the lubber's line of the compass, gyro repeater, and pelorus with the fore-and-aft line of the ship isof major importance. Such a misalignment will produce a constant Aerror in the curve of deviations. All of these instruments

may be aligned correctly with the fore-and-aft line of the ship by using the azimuth circle and a metal tape measure. Should

the instrument be located on the centerline of the ship, a sight is taken on a mast or other object on the centerline. In the case

of an instrument off the centerline, a metal tape measure is used to measure the distance from the centerline of the ship to the

center of the instrument. A similar measurement from the centerline is made forward or abaft the subject instrument and

reference marks are placed on the deck. Sights are then taken on these marks.

Standard compasses should always be aligned so that the lubber's line of the compass is parallel to the fore-and-aft line of

the ship. Steering compasses may occasionally be deliberately misaligned in order to correct for any magnetic A error

present, as discussed in article 411.

406. In addition to the physical checks listed in Chapter I, there are other precautions to be observed in order to assure

continued satisfactory compass operation. These precautions are mentioned to bring to the attention of the adjuster certain

conditions that might disturb compass operation. They are listed in Chapter I and are discussed further in Chapter X.Expeditious compass adjustment is dependent upon the application of the various correctors in an optimum sequence so as

to achieve the final adjustment with a minimum number of steps. Certain adjustments may be made conveniently at dockside

so as to simplify the adjustment procedures at sea.

407.Inasmuch as the Flinders bar is subject to induction from several of the other correctors and, since its adjustment is not

dependent on any single observation, this adjustment is logically made first. This adjustment is made by one of the following

methods:

(1) Deviation data obtained at two different magnetic latitudes may be utilized to calculate the proper length of Flindersbar for any particular compass location. Details of the acquisition of such data and the calculations involved are

presented in articles 605 to 609, inclusive.


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(2) If the above method is impractical, the Flinders bar length will have to be set approximately by:(a) Using an empirical amount of Flinders bar that has been found correct for other ships of similar structure.(b) Studying the arrangement of masts, stacks, and other vertical structures and estimating the Flinders bar length

required.

If these methods are not suitable, the Flinders bar should be omitted until data is acquired.

The iron sections of Flinders bar should be continuous and at the top of the tube with the longest section at the top.

Wooden spacers are used at the bottom of the tube to achieve such spacing.

408. Having adjusted the length of Flinders bar, place the spheres on the bracket arms at the best approximate position. If thecompass has been adjusted previously, place the spheres at the best position as indicated by the previous deviation table. In

the event the compass has never been adjusted, place the spheres at midposition on the bracket arms.

409. The next adjustment is the positioning of the heeling magnet by means of a properly balanced dip needle, as discussed

in Chapter XI.

410. These three adjustments at dockside - Flinders bar, spheres, and heeling magnet - will properly establish the conditions

of mutual induction and shielding on the compass, such that a minimum of procedures at sea will complete the adjustment.

411. Expected errors.Figure 318, "Summary of Compass Errors and Adjustment", lists six different coefficients or types of

deviation errors with their causes and corresponding correctors. A discussion of these coefficients follows:

The Aerror is more generally caused by the miscalculation of azimuths or by physical misalignments, rather than magnetic

effects of asymmetrical arrangements of horizontal soft iron. Thus, if the physical alignments are checked at dockside, and ifcare is exercised in making all calculations, the Aerror will be insignificant. Where an azimuth or bearing circle is used on a

standard compass to determine deviations, any observed Aerror will be solely magnetic Aerror. This results from the fact

that such readings are taken on the face of the compass card itself rather than at the lubber's line of the compass. On a

steering compass where deviations are obtained by a comparison of the compass lubber's line reading with the ship's

magnetic heading as determined by pelorus or gyro, any observed A error may be a combination of magnetic A and

mechanical A (misalignment). These facts explain the procedure wherein only mechanical A is corrected on the standard

compass by realignment of the binnacle, and both mechanical Aand magnetic Aerrors are corrected on the steering compass

by realignment of the binnacle (see article 405). On the standard compass, the mechanical Aerror may be isolated from the

magnetic Aerror by making the following observations simultaneously:

(1) Record a curve of deviations by using an azimuth (or bearing) circle. An Aerror found will be solely magnetic A.(2) Record a curve of deviations by comparison of the compass lubber's line reading with the ship's magnetic heading as

determined by pelorus or by gyro. Any Aerror found will be a combination of mechanical Aand magnetic A.

The mechanical Aon the standard compass is then found by subtracting the A found in the first instance from the total A

found in the second instance, and is corrected by rotating the binnacle in the proper direction by that amount. It is neither

convenient nor necessary to isolate the two types of Aon the steering compass and all Afound by using the pelorus or gyro

may be removed by rotating the binnacle in the proper direction by that amount.

The Berror results from two different causes, namely: the fore-and-aft permanent magnetic field across the compass, and a

resultant asymmetrical vertical induced effect forward or aft of the compass. The former is corrected by the use of fore-and-

aft B magnets, and the latter is corrected by the use of the Flinders bar forward or aft of the compass. Inasmuch as the

Flinders bar setting has been made at dockside, any Berror remaining is corrected by the use of fore-and-aft Bmagnets.

The C error has two causes, namely: the athwartship permanent magnetic field across the compass, and a resultant

asymmetrical vertical induced effect athwartship of the compass. The former is corrected by the use of athwartship C

magnets, and the latter by the use of the Flinders bar to port or starboard of the compass; but, inasmuch as this vertical

induced effect is very rare, the Cerror is corrected by athwartship Cmagnets only.

The Derror is due only to induction in the symmetrical arrangements of horizontal soft iron, and requires correction by

spheres, generally athwartship of the compass.

The existence of Eerror of appreciable magnitude is rare, since it is caused by induction in the asymmetrical arrangementsof horizontal soft iron. When this error is appreciable it may be corrected by slewing the spheres, as described in Chapter VI.

As has been stated previously, the heeling error is most practically adjusted at dockside with a balanced dip needle. (See

Chapter XI.)

412.A summary of the above discussion reveals that certain errors are rare, and others have been corrected by adjustments at

dockside. Therefore, for most ships, there remain only three errors to be corrected at sea, namely the B, C, and D errors.

These are corrected by the use of fore-and-aft Bmagnets, athwartship Cmagnets, and quadrantal spheres respectively.


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413.Study of adjustment procedure. Inspection of the general B, C, and Dcombination of errors, pictured in figure 413 will

reveal that there is a definite isolation of the deviation effects on cardinal compass headings.

Figure 413 B, C, and Ddeviation effects

For example, on 090 or 270 compass headings, the only deviation which is effective is that due to B. This isolation, and

the fact that the Beffect is greatest on these two headings, make these headings convenient for Bcorrection. Correction of the

B deviation on a 090 heading will correct the B deviation on the 270 heading by the same amount but in the oppositedirection and naturally, it will not change the deviations on the 000 and 180 headings, except where B errors are large.

However, the total deviation on all the intercardinal headings will be shifted in the same direction as the adjacent 090 or

270 deviation correction, but only by seven-tenths (0.7) of that amount, since the sine of 45 equals 0.707.

The same convenient isolation of effects and corrections of Cerror will also change the deviations on all the intercardinal

headings by the seven-tenths rule, as before. It will now be observed that only after correcting the Band C errors on the

cardinal headings, and consequently their proportional values of the total curve on the intercardinal headings, can the Derror

be observed separately on any of the intercardinal headings. The Derror may then be corrected by use of the spheres on any

intercardinal heading. Correcting Derror will, as a rule, change the deviations on the intercardinal headings only and not on

the cardinal headings. Only when the Derror is excessive, the spheres are magnetized, or the permanent magnet correctors

are so close as to create excessive induction in the spheres will there be a change in the deviations on cardinal headings as a

result of sphere adjustments. Although sphere correction does not generally correct deviations on cardinal headings, it does

improve the stability of the compass on these headings.

414.If it were not for the occasional Aor Eerrors which exist, the above procedure of adjustment would be quite sufficient,

i.e., adjust observed deviations to zero on two adjacent cardinal headings and then on the intermediate intercardinal heading.

However, figure 414, showing a combination of A and B errors, will illustrate why adjusting procedure must include

correcting deviations on more than the three essential headings.

If the assumption were made that no Aerror existed in the curve illustrated in figure 414, and the total deviation of 6E on

the 090 heading were corrected with Bmagnets, the error on the 270 heading would be 4E due to Bovercorrection. If then,

this 4E error were taken out on the 270 heading, the error on the 090 heading would then be 4E due to Bundercorrection.

Figure 414 A and B deviation effects


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The proper method of eliminating this to-and-fro procedure, and also correcting the Berror of the ship to the best possible

flat curve, would be to split this 4E difference, leaving 2E deviation on each opposite heading. This would, in effect correct

the Berror, leaving only the Aerror of 2E which must be corrected by other means. It is for this reason that, (1) splitting is

done between the errors noted on opposite headings, and (2) good adjustments entail checking on all headings rather than on

the fundamental three.

415.Before anything further is said about adjustment procedures, it is suggested that care be exercised to avoid moving the

wrong corrector. Not only will such practice be a waste of time but it will also upset all previous adjustments and

calculations. Throughout an adjustment, special care should be taken to pair off spare magnets so that the resultant field aboutthem will be negligible. To make doubly sure that the compass is not affected by stray fields from them, they should be kept

at an appropriate distance until one or more is actually to be inserted into the binnacle.

416. Adjustment procedures at sea. Before proceeding with the adjustment at sea, the following precautions should be

observed:

(1) Secure all effective magnetic gear in the normal seagoing position.(2) Make sure the degaussing coils are secured, using the reversal sequence, if necessary.

The adjustments are made with the ship on an even keel, swinging from heading to heading slowly, and after steadying on

each heading for at least 2 minutes to avoid Gaussin error (article 1003). Chapter VII discusses methods of placing a ship on

the desired heading.

417. Most adjustments can be made by trial and error, or by routine procedure such as the one presented in Chapter I.

However, it is more desirable to follow some analytical procedure whereby the adjuster is always aware of the magnitude ofthe errors on all headings as a result of his movement of the different correctors. Two such methods are presented:

(1) A complete deviation curve can be taken for any given condition, and an estimate made of all the approximatecoefficients. See Chapter V for methods of making such estimates. From this estimate, the approximate coefficients are

established and the appropriate corrections are made with reasonable accuracy on a minimum number of headings. If the

original deviation curve has deviations greater than 20, rough adjustments should be made on two adjacent cardinal

headings before recording curve data for such analysis. The mechanics of applying correctors are presented in figure

103. A method of tabulating the anticipated deviations after each correction is illustrated in figure 417a. The deviation

curve used for illustration is the one that is analyzed in Chapter V. Analysis revealed these coefficients:

A= 1.0E B= 12.0E C= 8.0E D= 5.0E E= 1.5E

Heading by

compass

Original

deviation

curve

Anticipated

curve after

first correcting

A= 1.0E

Anticipated

curve after

next

correcting

B= 12.0E

Anticipated

curve after

next

correcting

C= 8.0E

Anticipated

curve after

next

correcting

D= 5.0E

Anticipated

curve after

next

correcting

E= 1.5E

000 10.5E 9.5E 9.5E 1.5E 1.5E 0.0

045 20.0E 19.0E 10.6E 5.0E 0.0 0.0

090 11.5E 10.5E 1.5W 1.5W 1.5W 0.0

135 1.2W 2.2W 10.6W 5.0W 0.0 0.0

180 5.5W 6.5W 6.5W 1.5E 1.5E 0.0

225 8.0W 9.0W 0.6W 5.0E 0.0 0.0

270 12.5 W 13.5W 1.5W 1.5W 1.5W 0.0315 6.8W 7.8W 0.6E 5.0W 0.0 0.0

Figure 417a Tabulating anticipated deviations - Analysis method.


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(2) More often it is desirable to begin adjustment immediately, eliminating the original swing for deviations and theestimate or approximate coefficients. In this case the above problem would be solved by tabulating data and anticipating

deviation changes as the corrections are made. Figure 417b illustrates such procedure. It will be noted that a new column

of values is started after each change is made. This method of tabulation enables the adjuster to calculate the new

residual deviations each time a corrector is changed, so that a record of deviations is available at all times during the

swing. Arrows are used to indicate where each change is made.

Heading by

compass

Original

deviation

curve

Anticipated

curve after

first correcting

A= 1.0E

Anticipated

curve after

next

correcting

B= 12.0E

Anticipated

curve after

next

correcting

C= 8.0E

Anticipated

curve after

next

correcting

D= 5.0E

Anticipated

curve after

next

correcting

E= 1.5E

000 ... 10.5E 2.5E 2.5E 1.5E 0.0

045 ... ... 6.4E 1.4E 0.4E 0.4E

090 11.5E 0.0 0.0 0.0 1.0W 0.5E

135 9.2W 3.6W 1.4E 0.4E 0.4E

180 ... 5.5W 2.5E 2.5E 1.5E 0.0

225 ... 0.0 5.6E 0.6E 0.4W 0.4W

270 ... 1.0W 1.0W 1.0W 2.0W 0.5W

315 ... 1.2E 4.4W 0.6E 0.4W 0.4W

Figure 417b Tabulating anticipated deviations - One-swing method.

Since the Berror is generally greatest, it is corrected first; hence, on a 090 heading the 11.5E deviation is corrected to

approximately zero by using fore-and-aft Bmagnets. A lot of time need not be spent trying to reduce this deviation to exactly

zero since the Bcoefficient may not be exactly 11.5E, and some splitting might be desirable later. After correcting on the

090 heading, the swing would then be continued to 135 where a 9.2W error would be observed. This deviation is recorded,

but no correction is made because the quadrant error is best corrected after the deviations on all four cardinal headings have

been corrected. The deviation on the 180 heading would be observed as 5.5W. Since this deviation is not too large and

splitting may be necessary later, it need not be corrected at this time. Continuing the swing to 225 a 0.0 deviation would be

observed and recorded. On the 270 heading the observed error would be 1.0W, which is compared with 0.0 deviation on

the opposite 090 heading. This could be split, leaving 0.5W deviation on both 090 and 270, but since this is so small itmay be left uncorrected. On 315 the observed deviation would be 1.2E. At 000, a deviation of 10.5E would be observed

and compared with 5.5W on 180. Analysis of the deviations on 000 and 180 headings reveals an 8.0E, Cerror, which

should then be corrected with athwartship Cmagnets leaving 2.5E deviation on both the 000 and 180 headings. All the

deviations in column two are now recalculated on the basis of such an adjustment at 000 heading and entered in column

three. Continuing the swing, the deviation on 045 would then be noted as 6.4 E. Knowing the deviations on all intercardinal

headings, it is now possible to estimate the approximate coefficient D. Dis 5.0E so the 6.4E deviation on 045 is corrected

to 10.4E and new anticipated values are recorded in another column. This anticipates a fairly good curve, an estimate of

which reveals, in addition to the Bof 0.5E which was not considered large enough to warrant correction, an Aof 1.0E and

an E of 1.5E. These Aand Eerrors may or may not be corrected, as practical. If they are corrected, the subsequent steps

would be as indicated in the last two columns. It will be noted that the ship has made only one swing, all corrections have

been made, and some idea of the expected curve is available.

418. Deviation curves. The last step, after completion of either of the above methods of adjustment, is to secure all

correctors in position and to swing for residual deviations. These residual deviations are for undegaussed conditions of the

ship, which should be recorded together with details of corrector positions on the standard Navy Form NAVSEA 3120/4 and

in the Magnetic Compass Record, NAVSEA 3120/3. Article 901 discusses the purposes of the various NAVSEA Record

Forms more fully. Navy Form NAVSEA 3120/4 is complete and desirable in the interest of improved Flinders bar correction

and shielding conditions. Figure 418 illustrates both sides of form NAVSEA 3120/4 with proper instructions and sample

deviation and Flinders bar data. Should the ship be equipped with degaussing coils, a swing for residual deviations under

degaussed conditions should also be made and data recorded on NAVSEA 3120/4.

On these swings extreme care should be exercised in taking bearings or azimuths and in steadying down on each heading

since this swing is the basis of standard data for that particular compass. If there are any peculiar changeable errors, such as

movable guns, listing of the ship, or anticipated decay from deperming, which would effect the reliability of the compass,

they should also be noted on the deviation card at this time. Chapter X discusses these many sources of error in detail. If the


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Flinders bar adjustment is not based on accurate data, as with a new ship, it would be well to exercise particular care in

recording the conventional Daily Compass Log data during the first cruise on which a considerable change of magnetic

latitude occurs.

Figure 418 Deviation table - NAVSEA 3120/4

419. In order to have a reliable and up-to-date deviation card at all times it is suggested that the ship be swung to check

compass deviations and to make readjustments, if necessary, after:

(1) Radical changes in magnetic latitude.(2) Deperming. (Delay adjustment several days, if possible, after treatment.)(3)

Structural changes.(4) Long cruises or docking on the same heading such that the permanent magnetic condition of the vessels haschanged.

(5) Magnetic equipment near the binnacle has been altered.(6) Reaching the magnetic equator, in order to acquire Flinders bar data. (See Chapter VI.)(7) At least once yearly, to account for magnetic decay, etc.(8) Appreciable change of heeling magnet position if Flinders bar is present.(9) Readjustment of any corrector.(10) Change of magnetic cargo.(11) Commissioning.

With such reasonable care, the compass should be a reliable instrument requiring little attention.


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CHAPTER V

TYPICAL DEVIATION CURVE AND THE ESTIMATION OF APPROXIMATE

COEFFICIENTS

501. Simple analysis.The data for the deviation curve illustrated in figure 501 is as follows:

Ship's compass heading DeviationN (000) .......... .......... ........... .......... .......... .......... ........... .......... ........ 10.5E

NE (045) ........... .......... .......... ........... .......... ........... .......... .......... ..... 20.0E

E (090) ........................................................................................... 11.5E

SE (135) ........................................................................................ 1.2W

S (180) ........................................................................................... 5.5W

SW (225) ....................................................................................... 8.0W

W (270) ......................................................................................... 12.5W

NW (315) ......... ........... .......... ........... .......... .......... ........... .......... .... 6.8W

Figure 501 Typical deviation curve and its individual components

Since A is the coefficient of constant deviation, its approximate value is obtained from the above data by estimating the

mean of the algebraic sum of all the deviations. Throughout these computations the sign of east deviation is considered plus,

and west deviation is considered minus.

8A= + 10.5 + 20.0 + 11.5 1.2 5.5 8.0 12.5 6.8

8A= + 42.0 34.0

8A= + 8.0

A= + 1.0 (1.0E)

SinceBis the coefficient of semicircular sine deviation, its value is maximum, but of opposite polarity, on 090 and 270

headings. The approximateBcoefficient is estimated by taking the mean of the deviations at 090 and 270 with the sign at

270 reversed.

2B= + 11.5 (+12.5)

2B= + 24.0

B= + 12.0 (12.0E)


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Similarly, since C is the coefficient of semicircular cosine deviation, its value is maximum, but of opposite polarity, on

000 and 180 headings; and the approximate Ccoefficient is estimated by taking the mean of the deviations at 000 and

180 with the sign at 180 reversed.

2C= + 10.5 + (+5.5)

2C= + 16.0

C= + 8.0 (8.0 E)

D is the coefficient of quadrantal sine deviation having maximum, but alternately opposite, polarity on the intercardinalheadings. Hence, the approximateDcoefficient is estimated by taking the mean of the four intercardinal deviations with the

signs at 135 and 315 reversed.

40 = (+20.0) + (+1.2) + (8.0) + (+6.8)

40 = + 20.0

D = + 5.0 (5.0E)

E is the coefficient of quadrantal cosine deviation having maximum, but alternately opposite, polarity on the cardinal

headings. Therefore, the approximate Ecoefficient is estimated by taking the mean of the four cardinal deviations with the

signs at 090and 270reversed.

4E = (+10.5) + (11.5) + (5.5) + (+12.5)

4E = + 6.0E = + 1.5 (1.5E)

These approximate coefficients are estimated from deviations on compass headings rather than on magnetic headings. The

arithmetic solution of such coefficients will automatically assign the proper polarity to each coefficient. Summarizing the

above we find the approximate coefficients of the given deviation curve to be:

A = 1.0E

B = 12.0E

C = 8.0E

D = 5.0E

E = 1.5E

Each of these coefficients represents a component of deviation that can be plotted as shown in figure 501. The polarity of

each component in the first quadrant must agree with the polarity of the coefficient. A check on the components in figure 501

will reveal that their summation equals the original curve. This method of analysis is accurate only when the deviations are

less than 20. The mathematical expression for the deviation on any heading, using the approximate coefficients, is:

Deviation = A + B sin + C cos + D sin 2+ E cos 2

(where represents compass heading)

The directions given above for calculating coefficients A and B are not based upon accepted theoretical methods of

estimation. Some cases may exist where appreciable differences may occur in the coefficients as calculated by the above

method and the accepted theoretical method. The proper calculation of coefficientsBand Cis as follows:

Letting D1, D2, .., D8be the eight deviation data, then

2 1B=

8(D2+ D4 D6 D8) +

4(D3 D7)

2 1C=

8(D2 D4 D6+ D8) + 4

(D1 D5)


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CHAPTER VI

CORRECTOR EFFECTS-INTERACTIONS BETWEEN CORRECTORS

601.Until now the principles of compass adjustment have been considered from a qualitative point of view. In general, this is

quite sufficient since the correctors need merely be moved until the desired amount of correction is obtained. However, it is

often valuable to know the quantitative effects of different correctors as well as their qualitative effects. Furthermore, as has

been stated previously, all the correctors are not completely independent of each other. Interaction results from the proximityof the permanent magnet correctors to the soft iron correctors, with appreciable induction effects in the latter. Consequently

any shift in the relative position of the various correctors will change their interaction effects as well as their separate

correction effects. Additional inductions exist in the soft iron correctors from the magnetic needles of the compass itself. The

adjuster should therefore be familiar with the nature of these interactions so as to evolve the best methods of adjustment.

602. Quandrantal sphere correction.Figure 602 presents the approximate quadrantal correctionavailable with different

sizes of spheres, at various positions on the sphere brackets, and with different magnetic moment compasses. These

quadrantal corrections apply whether the spheres are used asD,E, or combinationDandEcorrectors. Quadrantal correction

from spheres is due partially to the earth's field induction and partially to compass needle induction. Since compass needle

induction does not change with magnetic latitude but earth's field induction does, the sphere correction is not constant for all

magnetic latitudes. A reduction in the percentage of needle induction in the spheres to the earth's field induction in the

spheres will improve the constancy of sphere correction over all magnetic latitudes. Such a reduction in the percentage of

needle induction may be obtained by:(1) Utilizing a low magnetic moment compass (article 613).(2) Utilizing special spheroidal-shaped correctors, placed with their major axes perpendicular to their axis of position.(3) Using larger spheres farther away from the compass.

Figure 602

603. Slewing of spheres.Figure 603a is a convenient chart of determining the proper slewed position for spheres. The total

values of theDandEquadrantal coefficients are used on the chart to locate a point of intersection. This point directly locates

the angle and direction of slew for the spheres on the illustrated binnacle. This point will also indicate, on the radial scale, the

resultant amount of quadrantal correction required from the spheres in the new slewed position to correct for both DandE

coefficients. The totalDandEcoefficients may be calculated by an analysis of deviations on the uncorrected binnacle, or by

summarizing the uncorrected coefficients with those already corrected. The data in figures 602 and 603b will be useful in

either procedure.


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Figure 603a Slewing of quadrantal spheres

Figure 603b Quandrantal error from Standard Navy Flinders bar.

Example:A ship having a Navy Standard binnacle, with 7" spheres at 13" position athwartship and a 12" Flinders bar

forward is being swung for adjustment. It is observed that 4EDerror and 6EEerror exist with the spheres

in position. Since the spheres are athwartship, the totalEcoefficient for the ship is 6E, as observed. Figure602 indicates that the spheres in their present position are correcting 6E D error, hence the total D

coefficient of the ship and Flinders bar is 10E. Figure 603a indicates that 6E Eand 10E Dcoefficients

require slewing the spheres 15.5 clockwise from their present athwartship position. The resultant quadrantal

error is indicated as 11.7. Figure 602 indicates that the 7" spheres should then be moved to the 11" position

after slewing 15.5 clockwise so as to correct both the D and E errors. Use of this chart will eliminate

mathematical or trial-and-error methods of adjustment for quadrantal errors as well as quickly provide

information for physically moving the spheres.


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604. Corrector magnet inductions in spheres.Should a ship have spheres and many permanent Band Cmagnet correctors

close to the compass, there will be a condition of induction existing between these correctors which will require some

shuttling back and forth between headings while making adjustments. This situation can be improved by using larger spheres

further out, and by approximately setting the spheres before starting adjustments, as well as by using more magnets further

from the spheres and compass. Magnetized spheres, as well as magnetized Flinders bar, will not only cause some difficulty

during adjustment, but might introduce an unstable deviation curve if they should undergo a shake-down or change of

magnetic condition.

605. Quadrantal error from Finders bar. Figure 603b presents the approximate quadrantal error introduced by thepresence of the Standard Navy Flinders bar. Since the Flinders bar is generally placed in the forward or aft position, it acts as

a small minusDcorrector, as well as a corrector for vertical induced effects. This means that upon inserting the Flinders bar

in such a position, the regular spheres should be moved closer to correct for the increased plus Derror, or vice versa, if the

Flinders bar is removed. ThisDerror in the Flinders bar is due mostly to compass needle induction since the bar is small in

cross-section and is close to the compass. Since such needle induction is practically constant, the deviation effects on the

compass will change with magnetic latitudes because the directive force, H, changes. However, when balanced by sphere

correctors this is advantageous because it tends to cancel out the variable part of the sphere correction which is due to the

compass needle induction.

606. Flinders bar adjustment.As has previously been stated in Chapter II, it is generally impossible to place the correct

amount of Flinders bar without reliable data obtained in two widely separated magnetic latitudes. The placing of Flinders bar

by the use of an empirical amount, or by an inspection of the ship's structures, is merely an approximation method and

refinements will usually be necessary when data is obtained. There are several methods of acquiring and utilizing suchlatitude data in order to determine the proper amount of Flinders bar, hence an elaboration on the following items:

(1) The data necessary for calculation of Flinders bar length, and the conditions under which this data should beacquired.

(2) The best method of utilizing such data to determine the proper length of the Flinders bar.607. Data required for Flinders bar adjustment.The data required for correct Flinders bar adjustment consists of accurate

tables of deviations with details of corrector conditions at two different magnetic latitudes, the farther apart the better. See

figure 418 for an example of how such data is recorded on NAVSEA Form 3120/4. Should it be impossible to swing ship for

a complete table of deviations, the deviations on east and west magnetic headings would be helpful. On many occasions,

ship's log data is available, but is of little use for Flinders bar calculation because it is not reliable. The following precautions

should be observed when such data is to be taken in order to assure that observed deviation changes are due only to changes

in theHandZcomponents of the earth's field.

(1) Degaussing should be secured, by a reversal process if necessary, at both latitudes before data is taken.(2) If the ship has been docked or steaming on one heading for several days prior to the taking of these data, the

resulting temporary magnetism (Gaussin error) would create erroneous deviations. A shakedown on other headings

prior to taking data would reduce such errors.

(3) Deperming, structural changes, heavy gunfire, magnetic cargoes, etc., subsequent to the first set of data will makethe comparative results meaningless.

(4) Inasmuch as the data will not be reliable if the ship's permanent magnetism changes between the two latitudes, itwill likewise be unreliable if any of the binnacle correctors are changed, including the heeling magnet.

In the event that an intelligent approximation as to Flinders bar length cannot be made, then the deviations at the two latitudes

should be taken with no Flinders bar in the holder. This procedure would also simplify the resulting calculations.

608. Methods of determining Finders bar length.

(1) Having obtained reliable deviation data at two different magnetic latitudes, the changes in the deviations, if any, may

justifiably be attributed to an incorrect Flinders bar adjustment. E/W and N/S deviations are the ones that are subject to major

changes from such an incorrect adjustment. If there is no change in any of these deviations, the Flinders bar adjustment isprobably correct. A change in the E/W deviations indicates an asymmetrical arrangement of vertical iron forward or aft of the

compass, which requires correction by the Flinders bar, forward or aft of the compass. A change in the N/S deviations

indicates an asymmetrical arrangement of vertical iron to port or starboard of the compass, which requires correction by the

Flinders bar to port or starboard of the compass. This latter case is very rare, but can be corrected, as indicated in article 613.


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609.After determining the correct amount of Flinders bar by either method (1) or (2) above, the bar should then be inserted at

the top of the holder and the fore-and-aftBmagnets readjusted to correct the remaining Berror. Sphere adjustments should

likewise be refined. It is quite possible that on inserting the Flinders bar, no visible deflection of the compass will be

observed, even on an east or west heading. This should cause no concern because certain additional induction effects exist in

the bar from:

(1) The heeling magnet.(2) The existing fore-and-aft magnets.(3) The vertical component of the ship's permanent magnetic field.

610. Heeling magnet induction in Finders bar.Figure 610 presents typical induction effects in the Flinders bar for different

positions of heeling magnet. An adjuster familiar with the nature of these effects will appreciate the advantages of

establishing the Flinders bar and heeling magnet combination before leaving dockside. Deviations must also be checked after

adjusting the heeling magnet, if Flinders bar is present.

Figure 610 Induction effects in Flinders bar due to heeling magnet

611. Slewing of Flinders bar.The need for slewing the Flinders bar is much more rare than that for slewing spheres. Also,

the data necessary for slewing the Flinders bar cannot be obtained on a single latitude adjustment, as with the spheres.

Slewing the bar to some intermediate position is, in effect, merely utilizing one bar to do the work of two; one forward or aft,

and the other port or starboard.Article 608 explains that a change of the E/W deviations with changes in latitude indicates the need for Flinders bar

forward or aft of the compass; and a change of the N/S deviations with changes in latitude indicates the need for Flinders bar

to port or starboard of the compass.

A change of theBdeviations on magnetic E/W headings is used, as explained in article 608, to determine the proper

amount of Flinders bar forward or aft of the compass by calculating the constant c. If there is a change of the Cdeviations on

magnetic N/S headings, a similar analysis may be made to determine the proper amount of Flinders bar to port or starboard of

the compass by calculating the constantffrom:

H1tan C1H2tan C2 f= Z1Z2

when

= shielding factor (0.7 to 1.0 average).H1= earth's field,H, at 1st latitude.

C1= degrees C deviation at 1st latitude (magnetic headings).

Z1= earth's field,Z, at 1st latitude.

H2= earth's field,H, at 2nd latitude.

C2= degrees Cdeviation at 2nd latitude (magnetic headings).

Z2= earth's field,Z, at 2nd latitude.

Any value of thisf constant indicates the need for Flinders bar adjustment athwartship of the compass, just as a value of the

cconstant indicates the need for Flinders bar adjustment forward or aft of the compass. The fconstant curve in figure 608b is

used for the determination of this Flinders bar length. If f is negative, Flinders bar is required on the starboard side of the

binnacle.


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612.Should both cand f exist on a ship, the angular position for a Flinders bar to correct the resultant vertical induction

effects may be found by:

f ftan =

cor = tan-1

c

is the angle to slew the Flinders bar from the fore-and-aft axis. If candfare negative, the bar will be slewed clockwise

from the forward position; if cis negative andfis positive, the bar will be slewed counterclockwise from the aft position.

After so determining the angle to slew the Flinders bar from the fore-and-aft line, the total amount of Flinders bar

necessary to correct the resultant vertical induction effects in this position is found by:

r= c2+f 2

The constant r is then used on the c or f constant curve in figure 603b to determine the total amount of Flinders bar

necessary in the slewed position.

613. Compasses.Compasses themselves play a very important part in compass adjustment, although it is common belief that

the compass is only an indicating instrument, aligning itself in the resultant magnetic field. This would be essentially true if

the magnetic fields were uniform about the compass; but unfortunately magnetism close to the compass imposes non-uniform

fields across the needles. In other words, adjustment and compensation sometimes employ non-uniform fields to correct

uniform fields. Figure 613a indicates the difference between uniform and non-uniform field effects on a compass.

Figure 613a Magnetic fields across compass needle arrays

Such unbalanced torques, arising from non-uniform magnetic fields, create deviations of the compass which have higher

frequency characteristics. Compass designs include many combinations of different length needles, different numbers of

needles, and different spacings and arrangements of needlesall designed to minimize the higher order deviations resulting

from such non-uniform magnetic fields. Although compass design is rather successful in minimizing such deviations, it is

obvious that different compasses will be affected differently by the same magnetic fields. It is further stressed that, even withproper compass design, it is the responsibility of all adjusters to exercise care in applying correctors in order to create the

most uniform magnetic field possible. This is the basis for the rule that requires the use of strong correctors symmetrically

arranged as far away from the compass as possible, instead of weak correctors very close to the compass. In general it is

better to use larger spheres placed at the extremities of the brackets, equally distant from the center of the compass. Band C

permanent magnet correctors should always be placed so as to have an equal number of magnets on both sides of the

compass where possible. They should also be centered as indicated in figure 613b, if regular tray arrangements are not

available. The desire for symmetrical magnetic fields is one reason for maintaining a sphere of specified radius, commonly

called the magnetic circle, about the magnetic compass location. This circle is kept free of any magnetic or electrical

equipment.


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Figure 613b Arrangements of corrector magnets

The magnetic moment of the compass needle array is another factor in compass design that ranks in importance with the

proper arrangement of needles. This magnetic moment controls the needle induction in the soft iron correctors, as discussed

in articles 602 and 605, and hence governs the constancy of those corrector effects with changes in magnetic latitude. The7" Navy No. 1 alcohol-water compass has a magnetic moment of approximately 4000 cgs units, whereas the 7" Navy No.

1 oil compass has a magnetic moment of approximately 1650 cgs units. The lower magnetic moment compass allows

considerably less change in quadrantal correction, although the periods are essentially comparable because of the difference

in the compass fluid characteristics.

Other factors that must be considered in compass design are period, fluid, swirl, vibration, illumination, tilt, pivot friction,

fluid expansion, etc. These factors, however, are less important from an adjuster's point of view than the magnetic moment

and arrangement of needles, and are therefore not discussed further in this text.


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CHAPTER VII

SHIP'S HEADING

701. Ship's heading.Ship's heading is the angle, expressed in degrees clockwise from north, of the ship's fore-and-aft line

with respect to the true meridian or the magnetic meridian. When this angle is referred to the true meridian, it is called a true

heading. When this angle is referred to the magnetic meridian, it is called a magnetic heading. Heading, as indicated on a

particular compass, is termed the ship's compass heading by that compass. It is always essential to specify heading as trueheading, magnetic heading, or compass heading. In order to obtain the heading of a ship, it is essential that the line through

the pivot and the forward lubber's line of the compass be parallel to the fore-and-aft line of the ship. This applies also to the

peloruses and gyro repeaters, which are used for observational purposes.

702. Variation.Variation at any place is the angle between the magnetic meridian and the true meridian. If the northerly part

of the magnetic meridian lies to the right of the true meridian, the variation is easterly, and if this part is to the left of the true

meridian, the variation is westerly. The local variation and its small annual change are noted on the compass rose of all

navigational charts. Thus the true and magnetic headings of a ship differ by the local variation.

703. Deviation. As previously explained, a ship's magnetic influence will generally cause the compass needle to deflect from

the magnetic meridian. This angle of deflection is called deviation. If the north end of the needle points east of the magnetic

meridian, the deviation is easterly; if it points west of the magnetic meridian, the deviation is westerly.

704. Heading relationships.A summary of heading relationships follows:

(1) Deviation is the difference between the compass heading and the magnetic heading.(2) Variation is the difference between the magnetic heading and the true heading.(3) The algebraic sum of deviation and variation is the compass error.

The following simple rules will assist in naming errors and in converting from one heading expression to another:

(1) Compass least (less than magnetic heading), deviation east.Compass best (greater than magnetic heading), deviation west.

(2) When correcting (going from compass to magnetic to true), apply the sign algebraically (+East, West).When uncorrecting (going from true to magnetic to compass), reverse the sign (East, +West).

(3) When correcting, easterly errors are additive. This single rule can be used to recall all four cases:When correcting, easterly errors are additive; westerly errors are subtractive.

When uncorrecting, easterly errors are subtractive; westerly errors are additive.

Formed from the first letter of each key word in the correcting process (Compass, Deviation, Magnetic, Variation, True),

the sentence "Can Dead Men Vote Twice?" is useful in making conversions of heading data. Although the aforementioned

statement can be used for uncorrecting (going from right to left in the statement as written), the first letters of the key words

in the uncorrecting process are also used to develop a memory aid for uncorrecting.

Complete facility with such conversion of heading data is essential for expeditious compass adjustment procedure.

Typical heading relationships are tabulated below:

Compass heading 358, magnetic heading 003, deviation 5E.

Compass heading 181, magnetic heading 179, deviation 2W.

Compass heading 040, deviation 3E, magnetic heading 043.

Compass heading 273, deviation 2W, magnetic heading 271Magnetic heading 010, deviation 2E, compass course 008.

Magnetic heading 270, deviation 4W, compass course 274.

Magnetic heading 358, variation 6E, true heading 004

Magnetic heading 270, variation 6W, true heading 264

True heading 000, variation 5E, magnetic heading 355

True heading 083, variation 7W, magnetic heading 090.


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705. Use of compass heading and magnetic heading for adjustment. The primary object of adjusting compasses is to

reduce deviations (to make the magnetic heading and the compass heading identical, or as nearly so as possible). The two

methods of accomplishing this are as follows:

(1) Place the ship on the desired magnetic heading and correct the compass so that it reads the same as this magneticheading. This is the preferred method.

(2) Place the ship on the desired compass heading and determine the corresponding magnetic heading of the ship, andcorrect the compass so that it reads the same as this known magnetic heading. This method is used whenever it is

impractical to place the ship on a steady magnetic heading for direct correction.

In using the magnetic heading method, the deviations of the compass are easily observed as the difference between thecompass reading and the known magnetic heading of the ship. The difficulty in using this method lies in placing the ship on

the desired magnetic heading and holding the ship steady on that heading while adjustments are being made.

When using the compass heading method, the ship can easily be brought to any desired compass heading, but the difficulty

is in the determination of deviations. Further difficulty arises from the fact that the steersman is steering by an uncorrected

compass whose deviations are changing as the necessary adjustments are being made. Therefore, as each adjustment is being

made, the steersman should attempt to hold the ship steady on that heading by some means other than the compass being

corrected. Adjustments by this method are made as a series of approximations, for example:

Place the ship on any desired compass course, and correct the compass to read the corresponding magnetic heading. This

will probably leave the ship on a course other than the desirable cardinal and intercardinal headings for compass adjustment.

For accurate results, the above procedure should be repeated.

If the compass has no appreciable deviations, the deviations taken on compass headings will closely approximate those

taken on magnetic headings. However, as the magnitude of errors increases, there will be a marked difference between the

deviations taken on compass headings and those taken on magnetic headings.

706. Methods of placing ship on magnetic headings. A ship may be brought on a magnetic heading by reference to a

gyrocompass. The magnetic variation is applied to true heading to determine the gyro course which must be steered in order

to place the ship on the desired magnetic heading. If the gyrocompass has any error, it must be taken into consideration. It is

well to calculate all such problems through true headings, since shortcuts on this procedure frequently lead to errors.

Examples of such relationships are tabulated below:

To steer

magnetic

course

With variation True course With gyro error Heading per

gyro compass

000 6W 354 0 354180 10E 190 0 190

270 4W 266 1E 265

315 6E 321 2E 319

225 17W 208 2W 210

358 0 358 3W 001

The difference between gyro heading and magnetic heading will be constant on all headings as long as the gyrocompass

error is constant and the variation does not change. This gyrocompass error may be determined by a comparison of the

calculated true azimuth of the sun and the azimuth as observed on a synchronized repeater.

It is to be remembered that gyrocompasses have certain errors resulting from latitude and speed change as well as turning

errors, and that these errors are not always constant on all headings. For these reasons, the gyro error must be checked

constantly, especially if the gyro is being used to obtain data for determining residual deviation curves of the magneticcompass.

707.A ship may be placed on a magnetic heading by aligning the vanes of an azimuth circle with the sun over the topside

compass. The sun is a distant object whose azimuth tangle from the north) may be computed for any given time. Methods of

calculating sun's azimuths are discussed in Chapter VIII. By setting the line of sight of the vanes at an angle to the right (or

left) of the fore-and-aft line of the ship equal to the difference between the computed magnetic azimuth and the desired

magnetic heading of the ship, and then swinging the ship until the sun is aligned with the vanes, the ship will be on the

desired magnetic heading. Simple diagrams (as in figure 707) with the ship and sun drawn in their relative positions, will aid

greatly in the visualization of each problem. The azimuth circle must always be kept level while making observations,

particularly of celestial bodies.


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Figure 707 Azimuth circle set-ups

708.A distant object (10 or more miles away) may be use in conjunction with the azimuth circle for placing the ship on

magnetic headings, provided the ship stays within a small area. This procedure is similar to that used with the sun except that

the magnetic beading of the object is constant. With an object 11.4 nautical miles distant, a change in position of 400 yards at

right angles to the line of sight introduces an error of 1.

709.A pelorus may be used to place a ship on a magnetic heading using the sun's azimuth in much the same manner as with

the azimuth circle. Use of the pelorus has the further advantage in that the magnetic heading of the ship can be observed

continuously as the ship swings. Such a procedure would be as follows:

The forward sight vane is clamped to the dial at the value of the sun's magnetic azimuth, and the sight vanes are then

trained so the sun is reflected in the mirror. As the ship turns, the magnetic heading is always observed under the

forward lubber's line if the vanes are kept on the sun, and this will serve as a guide for bringing the ship onto any

desired magnetic heading. As the desired magnetic course is approached, the compass can be read and corrected,

even before that magnetic course is actually obtained; and a final check can be made when the ship is on the exact

course. The pelorus must always be kept in a level position while making observations, particularly of celestial

bodies.

710.A distant object can be used in conjunction with the pelorus, as with the azimuth circle, in order to place the ship onmagnetic heading, provided the ship stays within a small area. (See article 708)

711.Methods of determining deviations on a compass heading.The deviations on compass headings may be obtained by a

comparison of the calculated magnetic azimuth of the sun and the azimuth as observed on the compass by use of an azimuth

circle. Methods of calculating sun's azimuth are discussed in Chapter VIII. The ship is placed on the desired compass heading

and an azimuth of the sun taken of the face of the compass card. The difference in degrees between the observed azimuth and

the calculated magnetic azimuth of the sun is the deviation on that compass course.

712.The pelorus may also be used in conjunction with the sun's azimuth to obtain deviations on compass headings. The ship

is brought to the desired compass heading, and the forward sight vane is set on the calculated value of the sun's magnetic

azimuth. The sight vanes are then trained on the sun, and the magnetic heading of the ship is indicated under the forward

lubber's line of the pelorus. The difference in degrees between the compass heading and magnetic heading of the ship

indicated by the pelorus is the deviation on that compass course.

713.The azimuth circle or pelorus can be used in conjunction with ranges or a distant object to obtain deviations on compass

courses. The procedure is similar to that used with the sun. A range consists of any two objects or markers, one in the

foreground and the other in the background, which establishes a line of sight having a known magnetic bearing. The true

bearing of such a range is determined from a local chart; this true bearing is converted to the magnetic bearing by applying

the variation, corrected for annual change, as given on the chart. Multiple ranges consist of several markers in the background

and a single marker in the foreground, or vice versa. The ship is brought to the desired compass course and, at the instant of

crossing the line of sight of the range, a bearing is taken with the azimuth circle or pelorus. With the azimuth circle, the

difference in degrees between the observed bearing of the range on the face of the compass and the known magnetic bearing

of the range is the deviation on that compass course. If using a pelorus, the forward sight vanes are set to the magnetic


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CHAPTER VIII

AZIMUTHS

801. Azimuths of the sun.Since accurate compass bearings of the sun can readily be observed for comparison with the sun's

calculated true bearing or azimuth (for time, date, and place of the observer) to obtain the compass error, the sun is a valuable

reference point for compass adjustment. The azimuths of other celestial bodies can similarly be determined, but are less

practical for compass work because of the poor visibility of stars and the more variable time rates and declinations of themoon and planets. Hence, subsequent explanations concern themselves only with the sun and its azimuths.

802. Astronomical triangle.The azimuth of the sun at any instant and place of the observer is determined by solving the

astronomical triangle for azimuth angle, Z, using the observer's latitude and longitude and the celestial coordinates of the sun

for the time of the observation as taken from the Nautical Almanac to effect the solution.

The astronomical triangle is formed on the celestial sphere by:

(1) the elevated pole of the observer (the radial projection of the geographic pole of the observer according towhether his latitude is north or south);

(2) the zenith of the observer (the radial projection of the observer's position on earth); and(3) the celestial body.

803. Local hour angle, LHA. The Greenwich hour angle, GHA, of the sun as taken from the Nautical Almanac for the time

and date of the observation is combined with the observer's longitude to obtain the local hour angle, the angle at the elevated

pole between the local celestial meridian (the observer's meridian) and the hour circle of the sun, always measured westward

from 0 to 360.

westLHA = GHA

+ eastlongitude

Meridian angle, t, is sometimes used instead of local hour angle to express the angle at the elevated pole between the local

celestial meridian and the hour circle of the sun. The meridian angle, t, of the sun is the angle at the elevated pole measured

from the meridian of the observer to the hour circle of the sun eastward or westward from 0 to 180. Thus, tdenotes the

sun's position east or west of the local celestial meridian. When the sun is west of the meridian, tis equal to LHA; when east,

magnetic compass adjustment manual

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